The present invention relates to a mobile rib finder which is used to aid in fastening roofing materials to ribbed roof metal deck structures. The device can be used in the installation of screw and plate assemblies in roofs. The device enables the installer to locate the proper position to fasten the screw/plate assembly in the roofing material to secure the roofing material to a lower roof deck structure which is undulating in nature. In the past, it has been difficult to properly and consistently locate the proper alignment of the screw with the uppermost portion of the lower roof deck structure, since that structure is hidden by the presence of an upper layer of roofing material. The present invention thus overcomes this problem and enables the reliable placement of screw/plate assemblies in the desired position.